The present invention relates to a diffraction device and, more particularly, to a diffraction device for efficiently converting a wide-band radiation beam into a diffraction beam of a predetermined order.
As a conventional diffraction device for reflecting and diffracting incident light and generating reflected/diffracted light in a direction corresponding to a wavelength of the incident light, a surface relief type diffraction grating is frequently used.
FIG. 1 shows a grating shape of a relief type diffraction grating prepared by machine work and its use state. A stepwise grating structure 21 is formed on the surface of a diffraction grating layer 20 on a substrate 19. When monochromatic light 22 is incident in a direction almost perpendicular to the grating surface 21, and a diffracted light beam 23 emerges in a direction almost perpendicular to the grating surface 21, a diffraction efficiency of this diffraction grating is maximized, and a wavelength at this time is called a blazed wavelength.
FIG. 2 shows measurement results of the diffraction efficiency of the diffraction grating. Although there is a slight difference between TE light (solid curve) and TM light (broken curve) depending on a polarized state of incident light, high diffraction efficiencies can be obtained.
When the diffraction grating obtained by machine work is used for a conventional beam splitter, it has excellent characteristics. However, when a reflection type diffraction grating of this type is used in other applications, various problems are posed as in other diffraction gratings.
A case will be examined below wherein the reflection type diffraction grating is disposed on an optical path bending mirror of a head-up display apparatus. In this case, a light beam incident on the diffraction grating and a light beam reflected and diffracted by the diffraction grating have a given spatial range so as to display a two- or three-dimensional image.
As a result, in order to spatially separate the light beam incident on the diffraction grating and the light beam reflected and diffracted by the diffraction grating, a certain angular separation is necessary between the two light beams.
A similar problem is posed when a diffraction grating element is used as an optical communication component. In terms of the arrangement of the entire apparatus, incident light and diffracted light must be spatially separated.
The drawback of the conventional diffraction grating is a decrease in diffraction efficiency when an angle defined by the incident light and the diffracted light is increased. For example, in FIG. 1, when an angle defined by the incident light 22 and the reflected/diffracted light 23 is increased, a certain light component is lost at the corner of the grating inclined surface 21, thus decreasing the diffraction efficiency.
The drawback of the relief diffraction grating obtained by machine work is difficulty in etching the grating on a non-flat surface and in formation of a non-linear grating. When the diffraction grating of this type is used for a spectroscope, the diffraction grating must be formed on a concave surface or the grating itself must have focusing characteristics in order to improve image formation performance of the diffraction grating itself. However, in order to achieve these requirements using machine work, machine control with higher precision than in a normal diffraction grating working technique is required, and this is impossible to achieve in practice.
As a method of solving the problem in machine work, a method of forming a diffraction grating by a holographic exposure method is known. In this method, a substrate coated with a photosensitive material such as a photoresist is exposed with two light beams from a laser to form interference fringes, thus preparing a diffraction grating.
A diffraction grating prepared by the holographic exposure method has many advantages. That is, the diffraction grating can be formed on an arbitrarily curved surface such as a concave surface, convex surface, and the like, and can have various aberration characteristics such as focusing characteristics. However, this diffraction grating prepared by the holographic exposure method has a problem in diffraction efficiency.
FIG. 3 shows diffraction efficiency characteristics of a diffraction grating prepared by the holographic exposure method. The diffraction efficiency has large polarization dependency, and TE light and TM light have different wavelengths corresponding to peak diffraction efficiencies. For this reason, the diffraction grating cannot be effectively utilized except for an optical system using perfectly linearly polarized light.